The effect of maxillary first molar derotation on arch length.

In the correction of Class II malocclusions, derotation of the maxillary first molars is required to obtain an idealized Class I relation. Because of its trapezoidal shape, the maxillary first molar is believed to provide an arch length gain attendant to its derotation, which may subsequently be used to resolve deficiencies mesial to it. Two commonly used mechanisms for maxillary first molar derotation and its associated centers of rotation were studied. A transpalatal arch, when accompanied by distal force equivalent at the level of the maxillary first molar center of resistance, can provide a center of rotation approximating the lingual attachment. This produces an arch length gain of 2.1 mm anterior to the molar, while simultaneously impinging on the maxillary second molar space 1.2 mm (per side). In reality, this may be a transference of a problem in one area of the dental arch to another. The 2 x 4 and 2 x 6 arch wires provide for various centers of rotation along an axis from the molar center of resistance--as projected to the occlusal surface--to the region of the maxillary first molar distal marginal ridge. These centers of rotation do not provide any meaningful alteration in arch length. These arch wires are useful, however, for maxillary first molar derotation necessary to achieve an idealized Class I molar occlusion. It is evident that the arch length gain related to derotation of the maxillary first molars, irrespective of the center of rotation, is insufficient to achieve Class I premolar intercuspation. Additional factors as growth, surgical intervention, or extractions are needed to achieve Class I premolar articulation.